The present invention relates to an improved method for removing impurities formed during liquid-liquid extraction operations in hydrometallurgical processing. Such impurities collect in the settling tank used to separate organic and aqueous phases after extraction has occurred.
Liquid-liquid extraction is commonly employed to recover and concentrate metal values from aqueous leach liquors that have been used to dissolve such metal values from their ores. In such a procedure, an aqueous leach liquor containing metal values in ionic form is thoroughly admixed in a mixing device or extraction column with an organic solvent containing an ion exchange or metal complexing agent. In admixture with the metal ion-containing aqueous leach liquor, the organic solvent medium selectively or preferentially dissolves and thereby extracts the metal values from the aqueous leach liquor. After organic solvent extraction of metal values has occurred to the extent desired, the organic/aqueous mixture or dispersion is fed to a settling tank or to the settling region of a mixer-settler wherein the metal-laden organic solvent extract separates by gravity from the metal-depleted aqueous solution. Metal values can be subsequently recovered from the metal-laden organic solvent extract by, for example, conventional electrowinning procedures.
When such an extraction process is conducted in a continuous manner, it is common to have a third phase liquid layer of impurities form between the organic and aqueous phases, i.e., at the organic/aqueous interface in the settling tank or region. Such a third phase impurities layer is colloquially referred to as "crud" in the hydrometallurgical industry. The "crud" layer consists primarily of the organic extraction solvent which contains smaller amounts of aqueous liquid and undissolved particulate material. Such a third phase "crud" layer which forms in the settling tank or region is actually a dispersion, e.g., an emulsion or suspension, of the aqueous and particulate matter in an organic matrix or continuous phase.
During continuous liquid-liquid extraction operations, the third phase layer of impurities accumulates at the organic/aqueous interface. This "crud" accumulation is such that in conventional extraction operations, the settling tank itself must be shut down, dumped and flushed out from time to time in order to prevent the accumulating layer of impurities from eventually interfering with the efficiency of the extraction procedure. Shut down of the settling tank or mixer-settler in the extraction process is, of course, economically disadvantageous, and it is thus an object of the present invention to provide a method for conveniently and economically eliminating the build-up of the third phase "crud" layer of impurities during the organic/aqueous separation procedure in a hydrometallurgical liquid-liquid extraction operation. It has been surprisingly discovered that this "crud" phase can be removed from an extraction settling tank or mixer-settler as it accumulates and broken into its organic, aqueous and particulate components by subjecting it to an electrostatic field.